Exposed pad integrated circuit package

ABSTRACT

An IC assembly including an exposed pad integrated circuit (“IC”) package having a thermal pad with a top surface and a bottom surface and with at least one peripheral surface portion extending transversely of and continuous with the bottom surface. The bottom surface and the at least one peripheral surface are exposed through a layer of mold compound. Also, methods of making an exposed pad integrated circuit (“IC”) package assembly. One method includes optically inspecting a solder bond bonding a thermal pad of an exposed pad IC package to a printed circuit board. Another method includes wave soldering an exposed pad of an IC package to a printed circuit board.

This application is a divisional of U.S. application Ser. No.14/671,727, filed Mar. 27, 2015, the contents of which are hereinincorporated by reference in its entirety.

BACKGROUND

Integrated circuit (“IC”) packages are often surface mounted on printedcircuit (“PC”) boards or other electrical connection boards. The ICpackages typically have components encased in a layer of protective moldcompound. Many modern IC packages have internal components, such as ICdies, that generate significant amounts of heat. One way of dissipatingheat generated by a die is to mount the die on one surface of a thermalpad that has the opposite surface exposed through the protective moldlayer. This exposed surface of the thermal pad is then attached to ametal layer plated on a PC board. Heat from the die is conducted throughthe thermal pad and into the PC board where it dissipates.

SUMMARY

This specification discloses an IC assembly of an integrated circuit(“IC”) package. The IC package includes a thermal pad having a topsurface and a bottom surface and at least one peripheral surface portionextending transversely of and continuously with the bottom surface. TheIC package also includes a layer of mold compound through which thebottom surface and the at least one peripheral surface are exposed.

Also disclosed are example embodiments of methods of: making an exposedpad integrated circuit (“IC”) package assembly; making an electricalassembly; and inspecting a solder bond bonding a thermal pad of anexposed pad IC package to a printed circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom isometric view of a prior art exposed pad integratedcircuit (“IC”) him package.

FIG. 2 is a cross-sectional elevation view of the prior art integratedcircuit package of FIG. 1.

FIG. 3 is a top isometric view of the thermal pad and leads of theintegrated circuit package of FIG. 1.

FIG. 4 is a detail cross-sectional side elevation view of the prior artintegrated circuit package of FIG. 1 solder bonded to a printed circuit(“PC”) board.

FIG. 5 is a top isometric view of one example embodiment of an exposedpad integrated circuit package.

FIG. 6 is a bottom plan view of the integrated circuit package of FIG.5.

FIG. 7 is a top isometric view of the thermal pad and leads of theintegrated circuit package of FIG. 5.

FIG. 8 is a detail cross-sectional side elevation view of the integratedcircuit package of FIG. 5.

FIG. 9 is a top plan view of a PC board.

FIG. 10 is a detail cross-sectional side elevation view of theintegrated circuit package of FIG. 5 solder bonded to a printed circuitboard.

FIG. 11 is a top isometric view of another example embodiment of anexposed pad integrated circuit package.

FIG. 12 is a bottom plan view of the integrated circuit package of FIG.11.

FIG. 13 is a top isometric view of the thermal pad and leads of theintegrated circuit package of FIG. 11.

FIG. 14 is a detail cross-sectional side elevation view of theintegrated circuit package of FIG. 11 solder bonded to a PC board.

FIG. 15 is a flowchart of an example embodiment of a method of making anexposed pad integrated circuit package.

FIG. 16 is a flowchart of an example embodiment of a method of making anelectrical assembly.

FIG. 17 is a flowchart of an example embodiment of a method ofinspecting a solder bond that bonds a thermal pad of an exposed pad ICpackage to a printed circuit board.

DETAILED DESCRIPTION

FIG. 1 is a bottom isometric view of a prior art exposed pad integratedcircuit package 10 and FIG. 2 is a cross-sectional elevation viewthereof. FIG. 3 is a top isometric view of a thermal pad 12 and leads 18of the integrated circuit package of FIG. 1. As shown in FIGS. 1-3, theexposed pad integrated circuit package 10 has a thermal pad 12 with atop surface 14, a bottom surface 16, front and rear edges 11, 13 andlateral side edges 15, 17 A plurality of leads 18 extend outwardly froma position near the lateral sides of the thermal pad 12. A die 20 havinga top surface 22 and a bottom surface 24 is bonded by a solder layer 28to the top surface 14 of the thermal pad 12. Bond wires 26 connectproximal ends of the leads 18 to electrical contacts (not shown) on thetop surface 22 of the die 20. In some embodiments the die 20 also has acontact surface on its bottom surface that is electrically connected tothe thermal pad 12 by the solder layer 28.

Front and rear tie bars 30 each have proximal ends 32 attached to thethermal pad 12 and have distal ends 34, which project forwardly andrearwardly, respectively. The thermal pad 12, leads 18, and bond wires26 are encapsulated in a block of mold compound 40. The block of moldcompound 40 has a top surface 42, a bottom surface 44, lateral sidesurfaces 46, 48, a front surface 50, and a rear surface (not shown). Aportion of each lead 18 projects laterally outwardly from a lateral side46, 48 of the mold compound block 40. A terminal end of each distal end34 of an associated tie bar 30 is flush with a corresponding front 50 orrear surface (not shown) of the mold compound block 40.

FIG. 4 is a detail cross-sectional side elevation view of the integratedcircuit package of FIGS. 1-3 solder bonded to a printed circuit (“PC”)board 60. The printed circuit board 60 has a top surface 62, whichincludes metal surface portions 64, etc. The bottom surface 16 of thethermal pad 12 is bonded to the metal surface portion 64 by a layer ofsolder 70 having a front edge 72. Because the solder layer 70 is coveredby the IC package 10 it is not possible to visually inspect the solderlayer 70. Thus, if the solder layer 70 is inspected at all, it is doneby x-ray. X-ray inspection of solder joints is more expensive thanvisual inspection and is considerably less convenient.

The inventors have developed new exposed pad IC packages, e.g., packages110 and 210 shown in FIGS. 5 and 11, which enable optical inspection ofthe solder bond between an IC package and associated PC board or otherelectrical substrate on which it is mounted.

FIGS. 5 and 6 are top isometric and bottom plan views of an exampleembodiment of an exposed pad integrated circuit package 110. FIG. 7 is atop isometric view of the thermal pad and leads of the integratedcircuit package 110. As shown by FIGS. 5-7, the IC package 110 isencased in a block of mold compound 112 with top and bottom surfaces111, 113 and mirror image front and back surfaces 114 and mirror imagelateral side surfaces 115 A plurality of leads 116 project laterallyfrom the lateral sidewall surfaces 115 of the mold compound block 112.Package 110 has a thermal pad 118 with a top surface 120 and an exposedbottom surface 121. Thermal pad 118 has an upwardly extending frontflange 122 with an exposed front surface 123. The thermal pad 118 mayalso have an upwardly extending rear flange 124 having an exposed rearsurface 125, FIG. 6.

In the embodiment illustrated in FIGS. 5 and 6, tie bars 132, 134 areintegrally formed with and extend along the lateral sides of the thermalpad 118. As best shown in FIG. 5, these tie bars 132, 134 may projectlongitudinally outwardly of the exposed surfaces of flanges 122 and 124and thus project longitudinally outwardly from the front and rear facesof the compound block 112.

FIG. 8 is a detail cross-sectional side elevation view of the integratedcircuit package 110. As shown by FIG. 8, a die 130 having a top surface131 and a bottom surface 133 is bonded by a solder layer 147 to the topsurface 120 of the thermal pad 118. Bond wires 139 connect proximal endsof the leads 116 to electrical contacts on the top surface 131 of thedie 130. In some embodiments the die 130 also has a contact surface onits bottom surface that is electrically connected to the thermal pad 118by the solder layer 147.

FIG. 9 is a top plan view of a PC board 140 on which the integratedcircuit package 110 is mounted. A top surface 142 of the PC board 140has a metal pattern thereon including a large central metal pad portion144 that corresponds in length and width to the thermal pad 118 of theIC package 110. The central metal pad portion 144 is designed to besolder bonded to the thermal pad 118 of the IC package 110. The topsurface 142 also has a plurality of smaller pad portions 146, which areadapted to be attached to the leads 116. The front and rear end portions149, 150 of the central metal pad portion 144 may extend longitudinallybeyond the outermost smaller pad portions 146.

FIG. 10 is a detail cross-sectional side elevation view of theintegrated circuit package of FIG. 5 solder bonded to the printedcircuit board 140. A solder layer 147 bonds the PC board pad surface 144to the bottom surface 121 of the thermal pad 118. A solder fillet 148 isformed at the forward end of the solder layer 147 where the solder haswicked up the front surface 123 of the front flange 123. This solderfillet 148 is not visually blocked or obscured by the IC package 110 andis thus easily optically inspected by eye or a machine vision system.The configuration at the rear portion of the PC board/IC packageassembly may be identical to that of the front portion and is thus alsoeasily optically inspected.

FIGS. 11 and 12 are top isometric and bottom plan views of an exposedpad integrated circuit package 210 of another embodiment. FIG. 13 is atop isometric view of the thermal pad and leads of the integratedcircuit package of FIG. 5. As shown by FIGS. 11-13 the exposed pad ICpackage 210 comprises a mold compound block 212 having lateral surfaces213 from which laterally extending leads 216 project.

A thermal pad 218 has a top surface 220 and a bottom surface 221. Thebottom surface 221 is exposed and flush (coplanar) with a bottom surface214 of the mold compound block 212. In other embodiments the bottomsurfaces 221 and 214 are not flush. Thermal pad 218 has an upwardlyextending front flange 222 having exposed front surface 223 that isflush with a front surface 215 of the mold compound block 212. Anexposed rear flange 224 has a rear surface 225 that is flush with therear surface of the mold compound block 212.

A forwardly projecting tie bar 232 is integrally formed on a top edge ofthe front flange 222. This tie bar may project forwardly from the frontflange and thus projects outwardly with respect to the front surface 215of the mold compound block 212, as best shown in FIG. 11. A rearwardlyprojecting tie bar 234 integrally formed on a top edge of rear flange224 projects from a rear surface 225 of the mold compound block 212.

FIG. 14 is a detail cross-sectional side elevation view of theintegrated circuit package 210 solder bonded to the printed circuitboard 240, which may be identical to PC board 140 described withreference to FIG. 9. A solder layer 246 bonds a PC board central padsurface 244 to the bottom surface 221 of the thermal pad 218. A solderfillet 248 is formed at the forward end of the solder layer 246 wherethe solder has wicked up the front surface 223 of the front flange 222.This solder fillet 248 is not visually blocked or obscured by the ICpackage 110 and is thus easily optically inspected. The configuration atthe rear portion of the PC board/IC package assembly may be identical tothat of the front portion and is thus also easily optically inspected.

The manner in which the exposed pad IC packages 110 and 210 aremanufactured may be essentially identical to the manner in which aconventional exposed pad IC package 10 is manufactured, except for thestep of creating forward and rear solder wettable metal flange surfaces223, 225, which are exposed through the front and rear surfaces of amold compound block 212.

The manner in which the packages 110 and 210 are solder attached to a PCboard or other electrical substrate with conventional reflow solderingmay be similar to the attachment process for a conventional exposed padIC package 10. As with the prior art process the PC board has a layer ofsolder paste applied to the surface pattern thereof that is to beattached to the package 110 or 210. The IC package 110 or 210 is thenplaced with the thermal pad thereof in contact with the solder paste andthe assembly is moved into a reflow oven where the solder reflows andbonds the package 110 or 210 to the PC board. A significant differenceis that when attaching the new packages 110 and 210 a solder filletforms and wets the front and/or rear edge of the thermal pad and thesolder fillet is visible. With the prior art attachment method andstructure there is no visible solder fillet in the solder layer thatattaches the thermal pad to the PC board.

The manner in which the exposed pad packages such as 110 and 210 aresolder attached to a PC board or other electrical substrate by wavesoldering cannot be performed with a conventional exposed pad package.For wave soldering to work there must be an exposed thermal pad sidesurface portion (front and/or rear). The solder is deposited at thisexposed portion and then wicks underneath the thermal pad, soldering itto the PC board. With a conventional package 10 there is no such exposedthermal pad side surface. Thus, the inventors new exposed pad packageconfiguration makes possible wave soldering attachment of an exposed padIC package to a PC board.

In a wave soldering operation to attach an integrated circuit package,e.g., 110, to a PC board, e.g. 140 the bottom surface 113 of the moldcompound block 112 is attached to the top surface of the PC board 140 byan adhesive (not shown). The adhesive holds the integrated circuitpackage 110 in place on the PC board 140 as the PC board is movedthrough a wave soldering machine. Within the wave soldering machine (notshown), molten solder is pumped to form a standing wave in a moltensolder tank. The PC board 140 moved over the solder wave, which contactsthe components of the integrated circuit package 110 that are glued tothe PC board. The molten solder is wicked between the metal surfaces ofthe leads 116 of the integrated circuit package and the metal padsurfaces 146 on the PC board 140. The solder is also wicked by capillaryaction between the bottom surface 121 of the thermal pad 118 of the ICpackage and the top metal surface of central metal pad portion 144 ofthe PC board, forming a solder joint therebetween

FIG. 15 illustrates a method of making an exposed pad integrated circuitpackage. The method includes, as shown at block 401, attaching a die toa top surface of a thermal pad and, as shown at block 402, leaving atleast a portion of at least one transversely extending side portion ofthe thermal pad exposed during molding.

FIG. 16 illustrates a method of making an electrical assembly. Themethod includes, as shown at block 421, providing an exposed padintegrated circuit (“IC”) package and a printed circuit board. Themethod also includes, as shown at block 422, wave soldering an exposedsurface of a exposed thermal pad of the IC package to the printedcircuit board.

FIG. 17 illustrates a method of inspecting a solder bond bonding athermal pad of an exposed pad IC package to a printed circuit board. Themethod includes, as shown at block 431, providing an exposed pad ICpackage having a thermal pad bonded to a printed circuit board. Themethod also includes, as shown at block 432, optically inspecting thesolder bond.

Certain embodiments of exposed pad IC packages, which make possiblevisual inspection of a solder layer that attaches an exposed thermal padof the package to a an electrical substrate, have been expresslydescribed in detail herein, along with associated methodology.Alternative embodiments of such exposed pad IC packages and associatedmethodology will occur to those skilled in the art after reading thisdisclosure. It is intended for the appended claims to be construedbroadly to cover all such alternative embodiments, except as limited bythe prior art.

What is claimed is:
 1. An IC assembly comprising: an exposed padintegrated circuit (“IC”) package having: a thermal pad having a topsurface and a bottom surface and having at least one peripheral surfaceportion extending transversely of and continuous with said bottomsurface; and a layer of mold compound through which said bottom surfaceand said at least one peripheral surface are exposed.
 2. The IC assemblyof claim 1 further comprising: an electrical connection substrate havinga top surface portion; and a layer of solder that bonds said bottomsurface and said at least one peripheral surface portion of said thermalpad to said top surface portion and said at least one peripheral surfaceportion of said electrical connection substrate.
 3. The IC assembly ofclaim 2 wherein said electrical connection substrate comprises a printedcircuit board.
 4. The IC assembly of claim 2 wherein said layer ofsolder is applied by wave solder.
 5. The IC assembly of claim 2 whereinsaid layer of solder is a layer of reflowed solder.
 6. The IC assemblyof claim 1 wherein said at least one peripheral surface portionextending transversely of said bottom surface extends perpendicular tosaid bottom surface.
 7. The IC assembly of claim 1 wherein said at leastone peripheral surface portion comprises at least one of a front and arear peripheral surface portion.
 8. The IC assembly of claim 1 furthercomprising a plurality of leads extending laterally outwardly of saidthermal pad; and a die attached to said top surface of said thermal padhaving electrical contact surfaces thereon electrically connected tosaid plurality of leads.
 9. The IC assembly of claim 8 wherein saidplurality of leads extending laterally outwardly of said first andsecond lateral side edges of said thermal pad comprise outer terminalend portions having generally flat bottom surfaces that are generallyparallel to said bottom surface of said thermal pad.
 10. The IC assemblyof claim 9 further comprising solder bonded to said generally flatbottom surfaces of said outer terminal end portions of said plurality ofleads.
 11. The IC assembly of claim 1 wherein said thermal pad bottomsurface is generally flat and wherein said layer of mold compoundcomprises a generally flat bottom surface that is generally coplanarwith said generally flat bottom surface of said thermal pad.
 12. The ICassembly of claim 7 wherein at least one of said front and rearperipheral surface portions of said thermal pad comprise at least oneflange extending transversely of said top and bottom surfaces of saidthermal pad; said at least one flange having an exposed surface.
 13. TheIC assembly of claim 12 further comprising solder bonded to said exposedsurface of said at least one flange. wherein said solder bonded to saidat least one flange extends upwardly from a bottom portion of said atleast one flange.
 14. The IC assembly of claim 12 wherein said at leastone flange has an exposed surface that is connected to said bottomsurface of said thermal pad by an arcuate surface portion.
 15. The ICassembly of claim 12 wherein said at least one flange extends the fullwidth of said thermal pad.
 16. The IC assembly of claim 12 wherein saidat least one flange extends only a portion of the full width of saidthermal pad.